Recently, expectations for fuel cells have been rapidly increasing from the aspect of the global environmental protection and in view of the fact that it is advantageous to directly use hydrogen as fuel such that the energy conversion efficiency is high, and so forth.
Heretofore, the fuel cells have been utilized in the space development and the ocean development. Recently, however, the fuel cells have been extended to power sources replacing automobile engines, and household generators, so that possibilities of them to be widely used have been increased.
In short, the fuel cell is a device wherein fuel (reducing agent) and oxygen or air (oxidizing agent) are continuously supplied from the exterior to be reacted electrochemically, thereby to produce electrical energy. The fuel cells may be classified based on their operating temperatures, kinds of using fuel, applications and so forth. Recently, however, in general, they are roughly classified into five kinds, i.e. a solid oxide fuel cell (SOFC), a molten carbonate fuel cell (MCFC), a phosphoric acid fuel cell (PAFC), a polymer electrolyte fuel cell (PEFC) and an alkaline aqueous solution fuel cell (AFC), mainly depending on kinds of using electrolytes.
These fuel cells are of the type using hydrogen gas produced from methane etc. as fuel. Recently, there is also known a direct methanol fuel cell (DMFC) wherein a methanol aqueous solution is directly used as fuel.
Among such fuel cells, attention has been paid to the solid polymer fuel cell (hereinafter also referred to as the polymer electrolyte fuel cell: PEFC) having a structure wherein a solid polymer film is sandwiched between two kinds of electrodes, and further, these members are sandwiched between separators.
This PEFC is configured such that a unit cell is formed by disposing electrodes, such as an air electrode (oxygen electrode) and a fuel electrode (hydrogen electrode), on both sides of a solid polymer film, and this unit cell is sandwiched on both sides thereof between separators for the fuel cell.
For example, as the structure of the PEFC, there can be cited a structure wherein a fuel electrode and an air electrode each in the form of a catalyst layer having a thickness of 10 ìm to 20 ìm are formed on both sides of a polymer electrolyte having a thickness of 20 ìm to 70 ìm and they are unified together, then porous support layers (carbon paper, porosity: about 80%) are attached as collecting members on the outer sides of the catalyst layers, and further, they are sandwiched between separators (partition plates) each serving also as a feed passage for reaction gas such as hydrogen or oxygen.
In the foregoing PEFC, the fuel (hydrogen) and the oxidant (air) are isolated so as not to directly react with each other, and further, it is necessary to convey hydrogen ions (protons) produced at the fuel electrode to the side of the air electrode.
In the fuel cell that operates at an ordinary temperature (100° C. or lower) and wherein protons move in the solid polymer film, a thin film (thickness: about 50 ìm) having a perfluorocarbon sulfonic acid structure having a sulfonic group as an ion exchange group can be used as the solid polymer film, so that a compact cell can be produced.
In the foregoing PEFC, as its output performance, the high output density of 1–3 A/cm2, 0.6–2.1 V/unit cell and 2.1 W/cm2 can be obtained.
In general, this PEFC is in the form of a stack structure (also called a PEFC stack) wherein its electromotive force is increased to fit the purpose by stacking a plurality of unit cells each having electrodes arranged on both sides of a solid polymer film. However, like a fuel cell for a portable terminal, for example, there are also those instances where an electromotive force is not required so much, but it is required to be of the flat type and as thin as possible.
On the other hand, in general, the PEFC stack uses a separator having a structure wherein one side thereof is formed with a fuel gas feed groove for feeding fuel gas to one of adjacent unit cells, while the other side thereof is formed with an oxidant gas feed groove for feeding oxidant gas to the other of the adjacent unit cells. By this, fuel gas and oxidant gas are supplied along the separator surfaces.
As the PEFC separators, there are known a separator obtained by planing a graphite board and applying a grooving process thereto, a molded separator of a carbon compound obtained by kneading carbon into resin, a metal separator applied with a grooving process by etching or the like, a separator wherein the surface of a metal material is coated with anticorrosive resin, and so forth. These separators are each formed with a fuel gas feed groove and/or an oxidant gas feed groove according to requirements.
However, in case of the direct methanol fuel cell (DMFC) wherein a methanol aqueous solution is directly used as fuel, there has been raised a problem that the feeding of fuel by the foregoing conventional separator having the fuel gas feed groove for feeding fuel gas becomes uneven depending on places.
Particularly, it has been problematic in case of the direct methanol type with the flat type wherein a plurality of unit cells are arranged in a flat manner and electrically connected in series.
As described above, in recent years, the possibility has been increased for the fuel cells to be widely used and, in case of the PEFC, in addition to the general stack structure, there has also been required such one wherein the electromotive force is not required so much, but that is of the flat type and as thin as possible. Further, in case of the direct methanol-type and flat-type PEFC, the problem of uneven fuel feeding depending on places can not be sufficiently solved and its countermeasure has been sought for.